A wireless communication system includes at least one User Equipment (UE). The UE may be fixed or have mobility, and may be called a different term such as a Mobile Station (MS), a User Terminal (UT), a Subscriber Station (SS), a wireless device, etc. Generally, a base station denotes a fixed station for communicating with a terminal, and may be called a different term such as a Node-B, a Base Transceiver System (BTS), an Access Point (AP), etc.
For efficient system configuration, a wireless communication system has a cell structure. A cell denotes an area obtained by subdividing a wide region into small areas in order to efficiently use a frequency. Generally, a base station is installed at the center of a cell to relay a terminal, and a cell denotes a service area provided by one base station.
When a neighbor cell of a wireless communication system uses the same subcarrier under a multi-cell environment, a reason of interference may be caused to users. This is called an inter-cell interference. More particularly, the inter-cell interference is problematic much to a UE located in the neighborhood of a cell boundary, Transmission from a UE to a base station is called an uplink (UL), and transmission from a base station to a UE is called a downlink (DL). In the DL, a UE located in the neighborhood of a cell boundary receives strong interference from a neighbor cell. In addition, in the UL, a UE located in the neighborhood of a cell boundary not only has a strong interference on a neighbor cell but also has a low transmission rate due to a path loss at a serving cell.
To reduce an inter-cell interference, neighbor cells may be allowed to use different subcarriers, but in this example, a radio resource that may be used by one base station reduces.
A multi-cell cooperative scheme has been proposed to reduce an inter-cell interference under a multi-cell environment. When the multi-cell cooperative scheme is used, a communication performance of a terminal located in the neighborhood of a cell boundary may improve. In this context, various discussions regarding a data transmission and processing method using a multi-cell cooperative scheme are in progress. One of representative schemes is a joint processing scheme of a cooperative Multi-Input Multi-Output (MIMO) type via data sharing. The joint processing scheme may transmit data instantaneously and simultaneously from each base station performing the multi-cell cooperative scheme to a UE, and the UE coherent-combines a signal received from each base station to improve reception performance.
Recently, as a demand for various wireless services increases rapidly, the conventional Radio Frequency (RF) resource used generally is exhausted gradually. As an alternative for this trend, an interest is concentrated on a millimeter wave (mmW) band that has been limited and used for a specific area such as celestial bodies, satellites, a military purpose, etc.
A millimeter wave denotes a radio wave having a wavelength between 1 millimeter and 10 millimeter, and corresponds to a radio frequency between 30 GHz and 300 GHz. According to International Telecommunications Union (ITU), this frequency belongs to an Extremely High Frequency (EHF) band, and this radio wave has a unique propagation characteristic. For example, a millimeter wave has characteristics of having a higher propagation loss compared to a radio wave of a lower frequency, and not passing through an object such as a building, a wall, and a tree branch, and being more sensitive to absorption in the atmosphere, and refraction and diffraction due to a particle such as a raindrop in the atmosphere. In contrast, due to a short wavelength of a millimeter wave, more antennas may be concentrated on a relatively small area. This enables realization of a high gain antenna of a small size.
ITU defines a frequency band of 3 GHz˜30 GHz as a Super High Frequency (SHF). Several higher frequencies in the SHF have a characteristic similar to that of a radio wave in an Extremely High Frequency (EHF) band such as realization of a large propagation loss and a high gain antenna of a small size.
In a millimeter wave band, a very large number of spectrums may be used. For example, a frequency in the neighborhood of 60 GHz is typically known as a 60 GHz band, and is available as an unlicensed spectrum in most of countries. In the United States, 7 GHz (57 GHz˜64 GHz) of spectrums in the neighborhood of 60 GHz is allocated for an unlicensed use. In addition, on Oct. 16, 2003, Federal Communications Commission (FCC) issued a report titled Report and Order, and FCC allocated 12.9 GHz as a spectrum for high density fixed wireless service in the United States in the relevant report (71-76 GHz, 81-86 GHz, and 92-95 GHz (Here, 94.0-94.1 GHz is used for federal government and so excluded)).
A 71-76 GHz band, a 81-86 GHz band, and a 92-95 GHz band are collectively denoted by an E band. Frequency allocation in the E band is a largest spectrum allocation by FCC, and is fifty times-greater than the entire cellular spectrum.
A millimeter wave band has abundant frequency resources and can use a wide bandwidth, so that the millimeter wave band is advantageous in high speed transmission, and more particularly, the millimeter wave band is used much for transmitting high quality moving images.
Millimeter wave communication using millimeter waves requires beam-forming of Tx/Rx due to the characteristic of the above-described millimeter wave. To apply a multi-cell cooperative scheme to the millimeter wave communication, a UE in a multi-cell cooperative system should simultaneously receive beams transmitted from different directions from a plurality of base stations. However, since one array antenna may receive only a beam of one direction having a maximum Rx antenna gain, a plurality of array antennas should be installed to the UE in order to apply the general multi-cell cooperative scheme to the millimeter wave communication. However, in this example, it is not easy to install a plurality of array antennas due to a limitation in an installation space of the UE and manufacturing costs.
In addition, in example of widening a reception range of one array antenna to simultaneously receive a multi-beam transmitted from a plurality of directions, an Rx antenna gain reduces and accordingly a reception performance and coverage reduce.
In addition, to realize a multi-cell cooperative system, synchronization between base stations within a Cyclic Prefix (CP) needs to be maintained. Since a CP length reduces to 1/10 compared to 4G due to reduction of a coherent time in a millimeter wave band, a separate high performance synchronization system should be established in order to reduce a synchronization error between base stations. Accordingly, the number of base stations that may be included in the multi-cell cooperative system is greatly limited.